“Er, think the timing was a bit off there,” whispers one of the concerned grips while Lewis, who famously starred in the Spielberg-produced TV series Band of Brothers, is dusted down and readied for another heart-stopping take.

The set-up is relatively simple. The film, the first for Lewis’s 33-year-old writerdirector brother Gareth, has the elder Lewis starring as Milo, a world-weary hitman about to swap a life of homicidal mayhem and chaos for the bucolic pleasures of the Welsh countryside. His merciless paymasters, however, have other ideas.

Thus, in today’s scene, Milo is sitting in his favourite armchair when he spots the red laser “bead” from a sniper’s rifle snaking its way up his torso. At the last moment he leaps athletically to the floor while his chair, and subsequently his entire apartment, are pulverised by rifle fire, John Woo-style.

Unfortunately, in the attempt to get the best audience-pleasing near-nanosecond reaction time, Lewis is either leaping too late or the squibs are detonating too early. By the third attempt anxious assistants are wondering if they’re going to run out of armchairs. While technicians are scurrying about the scene is dragging, the clock is ticking, lunch is fast approaching, and Lewis’s key co-star for the afternoon, Nickolaj Coster-Waldau (Wimbledon), can spare only a couple of precious hours before being whisked back to Norway for another movie.

And yet, strangely, miraculously even, there is no tension. Lewis simply jokes about the protective padding under his trousers, producers breeze in and out and nod approvingly at monitors while soundmen kick back and read novels between takes.

The mellow ambience is inspired partially by the fraternal relationship between actor and director. “We’ve been mucking about since we were kids,” says Gareth, a former television writer and award-winning short film-maker. “Pretending to be people we’re not. From a very early age we adopted two personae that would go off and have adventures. So this is all really second nature to us.”

“I’ve known Gareth for 33 years,” says Damian. “This project is very him. We always had a romantic notion that we’d make it together. But for a while it was just a pipe dream.”

Lewis, the searingly intense actor’s actor, has often seemed to display a certain ambivalence about screen stardom. A graduate of the classically focused Guildhall School of Music and Drama, he says that his inspirational references were John Gielgud and Laurence Olivier rather than the likes of Marlon Brando and Montgomery Clift. His choices since breaking cover in Band of Brothers have been both sparse and telling. He was impeccably icy in Stephen Poliakoff’s recent TV film Friends and Crocodiles, and is mesmerising as a twitchy American schizophrenic in the indie flick Keane (out this summer). He’s next up as a CIA officer in Philip Haas’s Iraq drama, The Situation.

When he claims that he’s not dazzled by the allure of bigbudget blockbusters, you believe him. “If we accept that actors have a fairly healthy dose of vanity,” he says, “then I would say that mine responds more to peer approval than it does to limelight.”

He adds that it wasn’t the need for peer approval or even brotherly coercion that brought him on board The Baker. It was ultimately, obviously, the script. “It’s all in the writing,” he says, describing how Milo eventually arrives in a tiny Welsh village, is mistaken for a baker and so begins his life anew. “OK, so the concept itself is quaint and charming, and in the best sense of those words. But it’s what Gareth’s done within the details of the story that makes it so exciting. It’s just superior to almost anything I’ve read — and I’ve read more scripts than all of us put together.”

Justin Williams, the film’s producer, agrees, casually revealing that they’ve already turned down advanced UK distribution deals, saving the finished product for more lucrative mainstream offers. “It’s a risk for us, obviously, but we want to wait and show it in its completed form,” Williams says. “It’s very commercial, and it doesn’t have a social statement tacked on to it. It’s driven purely by its humour, and we’ve got belief in that.”

Back on set, Gareth Lewis is shouldering the burden of this faith with ease. The armchair leap is nailed on the third take and everyone breaks for lunch. The script, the cast, the whole process, he muses, has been the experience of a lifetime. “Now, if I don’t f*** it up, we’ll all be happy,” he says.

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